SANTA CLARA — Cornerback Tramaine Brock will start Monday’s game for the 49ers — and he might not be budging from that lineup for several years. Brock signed a four-year, $16 million extension with $7 million guaranteed, a league source confirmed Friday.

Brock, 25, is in his fourth season after joining the 49ers as an undrafted free agent out of Belhaven (Miss.). He is earning $1.3 million this season on a one-year tender as a restricted free agent.

General manager Trent Baalke said in a release that Brock “has certainly earned this extension. Extending the contracts of our young and talented players has always been, and will continue to be, one of our primary goals.”

Brock’s stock has shot up since he took over as the 49ers’ No. 3 cornerback once Nnamdi Asomugha got hurt Sept. 22. Brock will start at right cornerback Monday night against the Washington Redskins in place of Tarell Brown, who sustained a rib injury in Sunday’s 23-20 loss at New Orleans.

Asked earlier Friday how Brock must adjust as a starter, defensive coordinator Vic Fangio said: “It’s amped up another level, he’s in there every play. He’s just got to be ready to play every play not knowing he’s got built-in breaks when we’re in base like he had for the first however many games he was playing. There is a change, but it’s just got to be a mental adjustment for him.”

Brock (5-foot-10, 197 pounds) leads the 49ers with four interceptions, the first of which he returned for a touchdown in an Oct. 6 win over the Houston Texans. His speed has also benefited the special-teams units.

If Carlos Rogers’ contract proves too expensive to retain, the 49ers’ cornerback corps next season likely would feature Brock, Chris Culliver and Eric Wright. Culliver, who replaced an injured Brock in 2011 as the No. 3 cornerback, is sitting out this season because of a knee injury sustained in training camp. Wright, a seventh-year veteran, made his season debut Sunday and is a pending free agent, as is Brown.

Rookie Darryl Morris is the only other cornerback under contract.

As the 49ers offense struggles in Colin Kaepernick’s first full season as the starting quarterback, offensive coordinator Greg Roman claimed Kaepernick is “ascending” and that recent struggles offer “a very opportunistic time for him to learn and get better.”

Kaepernick has been unable to lead the 49ers to 200 yards of total offense the past two losses, and he’s passed for over 200 yards just twice in 10 games this season.

“This is incredibly valuable to him, to go through these type of situations, battle his way out and learn from them,” Roman said. “Because to play quarterback in this league is a tall task. Over time, there are so many different things you’ll encounter, so many different players you’ll play with. All those different things that contribute to your growth as a quarterback.”

As coordinator of the league’s 29th-ranked offense in terms of yards per game, Roman believed the 49ers were a “few plays” away from avoiding their past two defeats, 10-9 to the Carolina Panthers and 23-20 at New Orleans. “We’re certainly not a finished product,” Roman said. “Those opportunities are out there, and the roof is going to blow off. It’s going to pop. It’s coming.”

The Oakland Raiders six-game winning streak ended and they fell out of the pole position in the AFC West with a 21-13 loss on Thursday Night Football to the Kansas City Chiefs. The Bay Area News Group’s Jimmy Durkin and Mark Purdy break down the loss from Kansas City, Mo.

Carr had the worst game of his young three-year career, at least statistically and probably overall. He completed just 17 of 41 pass attempts for 117 yards and zero touchdowns, giving him a passer rating of 49.1. In the football statistical world, that is the equivalent of playing Jeopardy and having Alex Trebek take away your buzzer. There were other...